Electronic organ



-w. F. KANNENBERG June 23, 1942 ELECTRONIC ORGAN Filed Dec. 7, 193's 6Sheets-Sheet 1- I INVENTOR M. F KANNENBERG ATTORNEY June 23, 1942. w. F.KANNENBERG 2,287,105

ELECTRONIC ORGAN Filed Dec. 7, 1939 6 Sheets-Sheet 2 INVENTOR a; a 3 I;as 8 5/. KAN/VENBERG A T TORNE Y June 23, 1942. w, KANNENBERG 2,287,105

ELECTRONIC ORGAN Filed Dec. 7, 1939 6 Sheets-Sheet 3 INVENTOR W EKANNENBERG BY A TTORNEV June 23, 1942. w F 4KANNENBERG 2,287,105

ELECTRONIC ORGAN 6 Sheets-Sheet 4 Filed Dec. 7, 1939 (Mt th #1 Q 0 Q QMthqQlOQ INVENTOR W F KANNENBERG BY A TTOR/VEV June 23, 1942. w. F.KANNENBERG 2,287,105

ELECTRONIC 0mm Filed Dec. 7, 1939 6 Sheets-Sheet .5

INVENTOR W E KANNENBERG BY A TTORNEV June 23, 1942- w. F. KANNENBERGELECTRONIC: ORGAN Filed Dec. 7, 1939 e Sheets-Sheet 6 v INVENTOR W EKANNENBERG A TTORNE'V Patented June 23, 1942 ELECTRONIC OB/GAN Walter F.Kannenberg, Rutherford, N. 1., assignor to Bell Telephone Laboratories,Incorporated, New York, N. 1., a corporation of New York ApplicationDecember 1, 1939, Serial No. 308,037

1: Claims. (01. 84-117) This invention relates to electronic organs ofthe type adapted to synthesize complex musical tones from a plurality ofrelatively pure sine.

wave sources.

The object of this invention is' to provide an improved means forcontrolling the tone quality or timbre of the synthesized complex soundwaves and to further embellish these synthesized complex waves by theaddition of combined Celeste, chorus and reverberation effects.

The foregoing object is attained by the apparatus of this inventionwhich comprises in combination an electronic organ, a quality controlmeans for modifying the quality of its output and a means based on theDoppler principle for further modifying the output by introducin undercontrol such special effects as have been variously termed chorus,celeste and reverberation effects.

The invention may be better understood by referring to the accompanyingdrawings in which:

Fig. 1 discloses a part of a conventional electronic organ manual;

Fig. 2 is one form of frequency generating system based on theutilization of equal-tempered harmonics;

Fig. 3 is a preferred form of harmonic or quality control adapted forfixed stops;

Fig. 4 shows in block form a suitable output system including means forintroducing special effects based on the Doppler principle;

Fig. 5 discloses a Doppler chamber in its preferred form;

Fig. 5A is a plan view of a section taken within the Doppler chambershown in Fig. 5;

Fig. 6 discloses two modified forms of quality control which may besubstituted for the form shown in Fig. 3;

Fig. 7 discloses another form of frequency generating system based onthe utilization of individual harmonic generators for each note on thekeyboard;

Figs. 8, 9 and 10 disclose other specific forms of harmonic controlbridge elements based on well-known principles which may be substitutedfor the form shown in Fig. 3; and

Fig. 11 is a key to assembling the several drawinas into an integratedwhole.

Referring now to Fig. 1 wherein a conventional keyboard I is disclosed,each key thereof having a movable contact 2 normally separated from astationary contact 3. Upon playing a key the contacts are engaged asindicated for keys C1 and E1. All of the movable contacts 2 areelectrically connected together and to one pole, for example, thepositive pole, of a direct current source oi supply 4 via conductors 5and G.

In order to provide thumpless" keying there is associated with thestationary contacts of each key, a potentiometer made up of tworesistors I, I, resistor 8 being in turn shunted by a condenser 9. Oneend of each of the resistors 8 and condensers 9 in the entire organ isconnected to ground strap l0 which is connected to the other pole of thedirect current power source 4 and to ground by conductor ll. When a keyis played the voltage across its associated resistor 0 will graduallyrise to a fixed limit with the charge taken by condenseril, and when thekey is released it will gradually fall to zero due to condenser 9discharging through resistor 8. The rates of rise and fall may bedifferent and are controlled in a well-known manner by the choice ofmagnitudes for resistors I, 8 and condenser v8. While this specific formof keying is disclosed for the sake of completeness, it will be obviousfrom what follows that any of the forms already well known in this artmay be substituted therefor provided they supply a direct currentvoltage for controlling the keying bridges of Fig. 2 or 7 hereinaftermore particularly described.

In Fig. 2 a, series of tone generators l 2 is shown each one generatinga substantially pure sine wave of frequency corresponding with thefrequency of one note in the equal-tempered scale. In the actualvorganthere will be as many such generators as there are equal-tempered notesof different frequencies in the organ or twelve for each chromaticoctave. In addition thereto there must be as many more in the audiblerange above the upper end of the keyboard as are necessary to provideharmonics for the notes which appear on the keyboard but whose audibleharmonics are above the notes appearing thereon. To avoid confusion onlyenough of these generators are herein disclosed to clearly explain theinvention. The output of each sine wave generator, for example, the C1generator, is supplied to a two-coil transformer I3, the secondary Hwhereof is center-tapped as shown. The center taps of all thesetransformers are connected together and to ground by a conductor IS.

The outer terminals l6 and ll of each secondvidual bridges are moreclearly shown in Fig. '1, the arrows being deleted in Fig. 2 forclarity. As many additional copper-oxide bridges are connected iinparallel to each of these bridges as are required to provide harmonicsfor the other notes on the keyboard. For example, the C: generatormust'have one bridge to provide an output when its associated key isplayed and must have another bridge to provide an output when the C1 keyis played because the frequency of C218 the second harmonic of C1.Likewise the G2 generator must have an additional bridge to supply thethird harmonic of C1, the Ca generator to provide the fourth harmonic,and so forth. This is true of nearly every note on the keyboard so thatthe number of bridgesper note will vary from one to seven for an organcapable of furnishing the first to'sixth and the eighth harmonics ofevery note played. If subharmonlc content is desired,

additional bridges are added in like manner. In Fig. 2 all bridgesconnected to the same generator are arranged in a row directly above it.

For the purposes of this invention, these bridges operate as follows:Each bridge is made up of four substantially identical arms containingnon-linear, asymmetric impedance elements such as copper-oxide rectifierunits poled as shown in Fig. 7 so that balance is ordinarily maintainedirrespective of the alternating cur-.

rent input from secondary ll. There is, accordingly, no alternatingcurrent output from the bridge to-the center-tapped primary it of outputtransformer l9. If, however, the center taps of secondary I4 and primaryi8 are connected to a suitable direct current source, an unbalance willappear so that the bridge will deliver an alternating current output tothe primary l8 of transformer is, the magnitude of which varies with theintensity of the direct current.

For convenience of quality control, all the bridges, producing thefundamental frequencies of their associated keys are arranged in onebank which may be called the fundamental bank. This is the lowerhorizontal row. of bridges in Fig. 2. All of the bridges which supplysecond harmonic contents are arranged in the next higher horizontal row,etc. The outputof each harmonic bank is collected by the secondaries 2|!of the output transformers I! in that bank.

These are serially connected as shown and one end thereof is connectedto ground while the other end of each is carried to their associated'quality control bridges in Fig. 3, via conductors 28 to II, inclusive.For the organ herein assumed, that is, one capable of supplying for eachnote the first to sixth, inclusive, and the eighth harmonics there willbe seven horizontal rows of such. bridges and, accordingly, seven groupsof transformer secondaries serially connected. In Fig. 2 these aredenoted F, 2F, IF, 41, SF, SF and SF, respectively, the seventh harmonicbeing omitted because it does not fall nearly enough on anyequal-tempered frequency to permit its inclusion without the addition ofconsiderable equipment. There is also an objection raised when a seventhharmonic is added to an equal-tempered instrument because of inherentdissonances. If additional harmonics or subharmonics are desired, it isonly necessary to provide an additional bank for each harmonic orderadded, the connections being made in the same manner as forthe bridgesshown.v

described is as follows: Assume that key C1 of Fig. 1 is played asindicated. A voltage will gradually rise across its resistor 8 until itreaches a predetermined limit depending upon the ratio of resistances ofresistors I and I, the capacity of the condenser Q and the voltage ofsupply 4. This voltage will be impressed via conductor 2| and groundupon the copper-oxide units of the C1 bridge in the fundamental or Fbank. It will also be impressed in like manner via conductor 22 upon theCa bridge in the second harmonic or 2F bank, via conductor 23 upon theGa bridge in the third harmonic or IF bank via conductor 24 to the Gabridge in the IF bank via conductor 25 to the E3 bridge in the BF bankvia conductor 26 to the G: bridge in the BF bank and finally viaconductor 21 to the C4 bridge in the 8F bank. Thus it is seen that whena given key is pressed all of the associated harmonic bridges are causedto deliver their full rated outputs into their output transformers. Theactual control over the proportioning of the harmonic content isattained in a manner to be hereinafter described.

Fig. 1 also shows the key E1 depressed and the circuit may be traced asbefore to find that the E1 bridge of the F bank, the Ea bridge of the 2Fbank, the B2 bridge of the SF bank, the E: bridge of the 4F bank, theGet bridge of the SF bank, the B3 bridge of the SF bank and the E4bridge of the BF bank are all activated. Circuits for all the other keysand bridges are not shown but are arranged in the manner indicated. forthese two keys. If no subharmonics are desired only one bridge will beconnected to each generator in the first or low bass octave. Ifsubharmonics of upper notes are desired then additional bridges areconnected to the upwardly extending'conductors l8, I1 and they arearranged in suitable. subharmonic banks in the same way as are theharmonic banks disclosed. In the second octave each generator will haveat least two bridges because, they furnish the second harmonic contentfor the first octave. All'the generators from G: to B2, inclusive, willhave an additional bridge in the third harmonic bank. All the bridgesare thus arranged, the largest number for any generator being 7 for theorgan assumed. Of course, if the lowest note on any manual is anychromatic note other than C1 the actual letters assigned to thegenerators in the circuits disclosed will have to be shifted thecorresponding number of chromatic intervals, there being no essentialstructural differences otherwise. The total number of bridges and theirarrangement in the complete organ is therefore obvious from the abovedescription and the accompanying drawings.

As previously stated, the outputs from the several harmonic banks arefed to harmonic or quality control bridges in Fig. 3 via conductors 28to 24, inclusive, and ground. Referring now to Fig. 3 it is seen thatthere is a row of seven bridges substantially like the bridges of Fig.2. There is one bridge for each harmonic F, 2F and so on to SF. Theinput transformers 35 in this case are located above the bridges andtheir primaries 36 are connected to their corresponding conductors 28 to24, inclusive, and to'ground. For example, primary II of the fundamentalbridge is connected to the serially connected secondaries 20 of thefundamental bank in Fig. 2 via conductor 22 and ground. The otherbridges are similarly connected to their respective har- The keyingoperation of the apparatus 80 far 9 W 111 8. 2.

The center-tapped secondaries 31 of these transformers are connected tothe inputs of the copper-oxide bridges the operation whereof being thesame as previously described for the bridges of Fig. 2. The directcurrent unbalancing potential for each bridge is supplied by aphotoelectromotive force cell 42 to the center taps on secondary 31 ofthe input transformer 35, and primary 39 of the output transformer 88.From the previous description it is clear that so long as any givenphotocell 42 produces no unbalancing voltage the correspondingalternating current input voltage will not be transferred to the outputcircuit. Thus, if all seven photocells are oark, there will be no outputeven though any number of the organ keys are played.

The amount of output transferred to the secondaries from any bridge willbe governed by the amount of illumination received by its associatedphotocell 42. For this purpose a separate cluster of lamps 43 areadapted to illuminate each cell. It is evident that the harmonic contentis thereb under control. Each lamp of the cluster 43 is fed from directcurrent source 4 of Fig. 1 via conductor 46, an individual resistor 44and a fixed stop tablet 45 to the grounded side of source 4. To imitatethe quality of a French horn, for example, the French horn tablet isoperated to connect one lamp of eachof the seven clusters to source 4through their individual resistors 44, the amount of each harmonic beingcontrolled by the size of its individual resistor. Other stops areprovided for other fixed qualities and they may be used singly or in anydesired combination for special effects as is well known.

The outputs from the seven output transformer secondaries 46 arecollected by serially connecting these secondaries together and to anorgan amplifier 49 via conductors 41, 48. The organ amplifier 49 isshown in block form in Fig. 4 and may be of any suitable design.

The description thus far describes the means for keying and the meansfor efiecting quality control. It is evident that the quality controlherein described is very flexible and exceedingly simple in itsoperation. The keying bridges in actual practice are of relatively smalldimensions and take very little space thereby lending themselves to verycompact console designs. The quality control bridges likewise are in noway bulky and obviously lend themselves to an extremely flexible controlover quality. Moreover, a very important result is attained by the useof this type of control, namely, silent keying and transition from onequality to another while playing. The only switch contacts used are atthe keys and stop tablets. The network formed by resistors 1, 8 andcondenser 9 as well as the copper-oxide keying bridge network affordsthumpless or clickless" keying while the natural build-up and decay timeof the lamp filament in cluster 43 affords silent quality transitionwhile playing.

As previously stated, the collected output from the secondaries of thequality control transformers are carried to the organ amplifier 49 ofFig. 4 via conductors 41 and 48. The output of organ amplifier 49 isamplified by a power amplifier 54 and fed to a speaker system 61 by wayof a swell control 65. In order to further modify and embellish theorgan output, special efiects variously termed as celeste, choral andreverberation are introduced by means of Doppler chamber 56. Theprincipal output of the organ amplifier 49 is fed to a mixer 58 by wayof conductors 50 and 5|. A diverted portion of this output is carried tothe Doppler amplifier by way of conductors 52 and 53 the output whereofis fed directly to the Doppler chamber 56 by way of conductors l8 and19. The special effects above enumerated are produced in this chamber ina manner to be explained more in detail in connection with Fig. 5.

The output of chamber 56 is amplified by pick-up amplifier 51 and fed tomixer 58 by way of conductors 59 and 60. portions the relative amountsof direct and diverted outputs from the organ amplifier 59. The outputof this mixer is carried to the power amplifier 54 by way of conductorsGI and 62,

The actual volume from the power amplifier 54 to reach the speakersystem 61 is under control of the swell control 65 which operatesessentially on the principle of an induction regulator. Since there areno contacts to be made or broken in this type of swell control, itintroduces no noise whatever in the speaker system. The output of thepower amplifier 54 is connected to the field windings ID and ll of theswell control '65 by means of conductors 63, 64. A wound rotor 12disposed in mutual inductive relation with field windings 70, II isadapted to be rotated through 90 electrical degrees by means of swellcontrol pedal 66. Rotor I2 is preferably wound in a conventional,distributed form such as commonly used on armatures. The mutualinductive relationship between field windings l6 and H and wound rotor12 is thereby made variable from substantially zero to its maximumvalue. The output from this rotor is carried to the speaker system byway of permanently attached conductors 68 and 69.

Since any changes in the mutual inductive relationship between woundrotor 12 and the field windings 10 and H changes the effectiveterminating impedance for the power amplifier 54, an auxiliaryterminating impedance i3 is connected to rotor '12 by way of conductorsand 8|. The impedance of auxiliary impedance I3 is substantiallyidentical with the impedance of coil 14 in the speaker system 61.Conductors 80 and 8| are connected to wound rotor 12 in such a manner asto be displaced substantially 90 electrical degrees from the connectionsmade by conductors 68 and 69 so that the portion of the rotor windingwhich is connected to conductors 68 and 69 is approaching a zero mutualinductive relationship with the field windings l0 and H while theportions of the rotor windings connected to conductors 86 and 8! areapproaching their maximum mutual inductive relationship. This maintainsthe eilective terminating impedance more nearly uniform throughout theoperative range of swell control. While pedal 66 is shown linked towinding 72, it is obvious that field 10, H may be the rotor whilewinding 12 is the stator in which case pedal 65 would be linked to thefield 10, H.

The actual physical apparatus employed for the various amplifiers,mixer, speaker system and the swell control 65 in Fig. 4 may be of anyconvenient design. A preferred form for the Doppler chamber 56, however,is disclosed in Fig. 5.

Fig. 5 discloses the Doppler chamber 56 in the form of a sound insulatedchamber having sound-proof walls 82. Within chamber 82 a rotatable disc85 is mounted on a shaft 86 adapted to be rotated by a gear reduc- Mixer58 pro-- sound-proof 4 while the latter connects to source 4via'co'ndoctor 8 in Fig. l. The upper end of shaft 88 is supported by abearing 81, while the lower end of shaft 88 is supported by a thrustbearing 88. Three directional type loud-speakers 88, 88, 88 aresymmetrically disposed around the outer periphery on the upper side ofdisc 88, while three similarly disposed loud-speakers 84, 84, 84 aremounted on the underside of disc 88. The output from the Doppleramplifier 58 of Fig. 4 is carried to these loud-speakers by way ofconductors 18 and I8. Conductor I8 connects to the loud-speakers by ,wayof conductor 8| through slip ring 88, while conductor I8 is connected tothe lower thrust bearing 88 and is carried to the loud-speakers by wayof shaft 88 and disc 85. In Fig. 5A immediately above Fig. 5, a planview is shown of the connections between the loud-speakers on the disc.Here it is seen that conductor 8| branches into conductors 92, 82, 92which connect the loud-speakers in parallel in groups of three.

Along one wall of chamber 82 and substantially in line with the edge ofdisc 85 there is -mounted a, microphone 88 adapted to pick up the directand reflected acoustic energy from loud-speakers 88 and 84. ConductorsI6 and I1 transmit the output from this microphone to the pick-upamplifier 51 as shown in Fig. 4.

In the actual use of this device, shaft 88 may be rotated at variousspeeds ranging from about fifty revolutions per minute to one hundredrevolutions per minute, depending upon the effect desired. For thispurpose the speed control device of conventional design is shown forcon-.

trolling the speed of the gear reduction motor 88. Moreover, shouldreverberation effects only be desired, the motor 88 is stopped whereuponchamber 56 acts only as a reverberation chamber in a manner well knownin the art. It frequently happens, however, that the organist desires toenhance the beauty of tone'by introducing such effects as are commonlytermed celeste or choral. These eflects are produced in a unique mannerby the apparatus or this invention which operates on the well-knownDoppler principle.

The apparent pitch of a sound source vibrating at a constant frequencyis higher when moving toward a receiver than when stationary and,conversely, is lower when moving away from the sound receiver. In Fig.5, for example, shaft 88 may be rotated in either direction. Assume, forexample-that it is rotated in the clockwise direction viewed from thetop. Neglecting for the moment the effect of the reflected sound wavesin the reverberation chamber, the direct sound waves received fromdirectional loud-speakers 83 by microphone 88 will cyclically vary bothin amplitude and apparent frequency as they rotate about shaft 88. Asthey approach microphone 88, their apparent pitch is higher andamplitude relatively high, while as they move away from microphone 88,their amplitude is relatively low and apparent pitch also low. The

same effect is produced by the microphones 84 on the underside oi disc88 except that their amplitude eilect is Just the'reverse, that is tosay, as the loud-speakers 84 recede from microphone 88. their relativevolume is greater due to their directional eilect than when they areapproaching microphone 88.

This rapid frequency shirt produces what is commonly referred to as the"celeste" eilect.

while this eilect combined with the fact that a plurality of speakersare used simultaneously produces the effect commonly termed the cho-.

ral";ell'ect. Ii the inner walls of chamber 82- were totally soundabsorbing, these effects would be producedwithout reverberation, butsince a certain amount oi. reverberation is usually desirable, thecharacter of the inner walls 0! chamber 82 are so selected and thedimensions thereof are so proportioned as to produce the desirablereverberation effect in the auditorium in which the organ is to beemployed. It has been found experimentally that the introduction of thereverberation efiect per se is insuillcient to produce the resultsdesired and that the introduction of a certain amount of celeste andchoral eflects simultaneously with the reverberation effect greatlyenhances the beauty of the music. The apparatus 01' this inventiontherefore provides a convenient means of obtaining all of these effectssimultaneously with a single piece of apparatus.

It is obvious. that where controlled reverberation content will not berequired, chamber 82 may be eliminated and the rotating speaker systemmay be located in the room with the main speaker system 81. In this casemicrophone 88, pick-up amplifier 61 and mixer 88 may or may not be usedas the requirements of the particular installation dictate. If they arenot used and the celeste and choral efiects are obtained by directacoustic radiation from the rotating speaker system, the swell control88 must be coordinated with a similar device associated with therotating speaker system. Such a modification is so obvious from a meremention thereof as to require no further discussion.

Fig. 6 discloses two modifications of the quality control bridgesdisclosed in Fig. 3. The first of these modifications is shown at theleft where the lamps of cluster 48 are all shown connected in parallel.These lamps as in the case of Fig. 3

are supplied with current from sourse 4 in Fig. 1.

The high voltage side of source 4 is connected to the lower end or thelamp cluster by means of conductor 48 while the grounded side of source4 is connected to the upper side of the lamp cluster by means ofconductor 88 and continuously variable rheostat 85. The symbol forrheostat 88 is intended to represent a substantially stepless resistancecontrol device so that the illumination intensity of lamp cluster 48 maybe continuously varied from substantially zero brilliance to maximumbrilliance.

The second modification shown on the right in Fig. 6 is adapted tosuccessively add the several lamps,or cluster 48 through separatecontacts 81 of the quality'control switch 88. This type of controlincreases the total brilliance on photocell 42 in successive, discretesteps.

The quality control scheme disclosed in Fig. 3 or either of the twomodifications disclosed in Fig. 6 may be employed either singly or incombination in any particular organ. That is to say, a series of fixedstops may be employed as shown in Fig. 3, and, in addition thereto, aseparate cluster of lamps and associated control means thereior inaccordance with either or both of the modifications shown in Fig. 8 mayalso be used. The fixed stops permit the organist to rapidly change fromone predetermined conventional tone quality to another while playing,while the variable quality controls shown in Fig.

6 permit him to employ a substantially infinite variety of tone colorsand vary them to suit his taste and fancy.

In Fig. 7 a modified arrangement for the keying bridge of Fig. 2 isdisclosed. In Fig. 2 only one set of generators were employed, thesebeing tuned to the equal-tempered chromatic scale while the harmonicsand subharmonics of any particular note in this scale were obtained onan equal-tempered basis from the other generators. In referring again toFig. 2 it will be noted that the individual sine wave generators l2 werecoupled to their associated keying bridges by transformer l3 havingcenter-tapped secondary windings H. In Fig. 7 these sine wave generatorsare used but have been deleted leaving only the center-tappedsecondaries l4.- Moreover, instead of having only one set of sine wavegenerators as in Fig. 2, Fig. '7 requires one sine wave generator foreach keying bridge. There will, therefore, be for Fig. 7 one sine wavegenerator for each keying bridge in the fundamental bank, another sinewave generator for each keying bridge in the second harmonic bank, etc.This arrangement makes it possible to employ the true harmonics insteadof the equal-tempered harmeans permanently coupling the bridge outputcircuits together to form the said separate harmonic order circuit and aquality control means for controlling the relative harmonic energydelivered from said separate harmonic order circuit to the outputsystem.

2. The combination of claim 1 wherein the quality control meanscomprises a normally balanced bridge network for each of the separateharmonic order circuits, means permanently coupling said bridge networkbetween its associated harmonic order circuit and the output system andcontrol means for controlling the degree of unbalance of each of thebridge networks.

3. In an electronic organ having an output system and a plurality ofelectric generators for producing a musical series of fundamental. andharmonic frequencies, a means for confining each harmonic order to aseparate harmonic order circuit, and a quality control means thereforcomprising a normally balanced bridge network for each of the separateharmonic order circuits, means permanently coupling said bridge networkbetween its associated harmonic order circuit and the output system andcontrol means for controlling the degree of unbalance of each of thebridge networks.

4. A swell control means for an electronic organ comprising a primarywinding and a secondary winding adapted for variable mutual inductivecoupling, input terminals for the priwherein Figs. 22, 24 and 26disclose respectively the Figs. 8, 9 and 10 of this application.Briefly, however, the bridge of Fig. 8 operates essentially like anordinary alternating current impedance bridge in which the arms are madeup of essentially pure resistances. The inherent capacity of photocellI2 is balanced out by means of a small variable condenser I00. Whenphotocell A2 is illuminated, its resistance changes thereby unbalancingthe bridge.

The bridges of Figs. 9 and 10 also operate like ordinary alternatingcurrent impedance bridges. In these cases, however, the unbalance iseffected by means of the non-linear characteristic of the copper-oxideunits in the bridge arms. The electrical outputs from the keying bridgesof F 2 enter these bridges by means of transformer 35, the outputvoltage whereof is sufficiently low to prevent the copper-oxide unitsfrom departing from the linear portions of their characteristics.However, when a direct current component is introduced by light fallingon photocell 42 sufiicient current is carried by the copper-oxide unitin opposite arms of Fig. 9 or by the copper-oxide unit in the arm ofFig. 10 to cause these units to depart from linearity therebyunbalancing the bridge.

What is claimed is:

1. In an electronic organ having an output system and a plurality ofelectric generators for producing a musical series of fundamental andmary winding, output terminals for the secondar winding, an artificialimpedance load having an impedance substantially equal to the out putcircuit to which the output terminals are to be connected, meansconnecting the artificial impedance load to the secondary at pointssubstantially electrical degrees from the output terminals and means forvarying the mutual inductive coupling between the primary and thesecondary windings.

5. A swell control means for an electronic organ comprising a primarywinding, a secondary winding wbund in distributed form and adapted forvariable mutual inductive coupling with said primary winding, inputterminals for the primary winding, output terminals for the secondarywinding connected thereto at points substantially electrical degreesapart, an artificial impedance load having an impedance substantiallyequal to the output circuit to which the output terminals are to beconnected, means connecting the artificial impedance load to thesecondary at points substantially 90 electrical degrees from the outputterminals, and means for varying the mutual inductive coupling betweenthe primary and the secondary windings.

6. In an electronic organ having a means for producing celeste andchoral effects comprising the combination of a soundproof chamber, adisc rotatable about the principal axis thereof nor-- mal to its plane,means for supporting said disc within said chamber, a directionalelectroacoustic transducer means rigidly mounted on said disc to face adirection substantially tangential to the circle formed thereby as thedisc is rotated and means for rotating said disc.

7. In an electronic organ having a means for producing celeste andchoral effects comprising the combination of a soundproof reverberationchamber, a disc rotatable about the principal .axis thereof normal toits plane, means for supporting said disc within said chamber, a direc-.

tional electroacoustic transducer means rigidly 8. In an electronicorgan having a main electrlc output channel, the combination comprisinga soundproof chamber, a disc rotatable about.

the principal axis thereof normal to its plane, means for supportingsaid disc within said chamber, a directional electroacoustic transducermeans rigidlymounted on said disc "to face a direction substantiallytangential to the circle formed thereby as the disc is rotated, meansfor rotating said disc, means for controlling the speed of said rotatingmeans, means for diverting someof the electric organ output from themain channel into said transducer means to be translated into acousticenergy, a pick-up means adapted to receive saidacoustic energy andtransform it back into electric energy, and means for combining. saidelectric energy and the undiverted portions in said main channel.

9. In an electronic organ having a main electric output channel, thecombination comprising a soundproof reverberation chamber, a discrotatable about the principal axis thereof normal to its plane, meansfor supporting said disc within said chamber, a directionalelectroacoustic transducer means rigidly mounted on said disc to face adirection substantially tangential to the circle formed thereby as thedisc is rotated, means for rotating said disc, means for controlling thespeed of said rotating means,

means for diverting some of the electronic organ output from the mainchannel into said transducer means to be translated into acousticenergy, a pick-up means adapted to receive said acoustic energy andtransform it back into electric energy, and means for combining saidelectric energywith the undiverted portion in said main channel.

10. The combination of claim 6 wherein the transducer means comprises aplurality of directional electro-acoustic transducers each rigidlymounted on said disc to face a direction substantially tangential to thecircle formed thereby as the disc is rotated.

11. The combination of claim '7 wherein the transducer means comprises aplurality of directional electro-acoustic transducers each rigidlymounted on said disc to face a direction substantially tangential to thecircle formed thereby as the disc is rotated.

12. The combination of claim 8 wherein the transducer means comprises aplurality of dithe disc is rotated.

WALTER F. KANNENBERG

